Self-precipitating compositions, which are also referred to as autophoretic baths in the technical jargon, are used for organic coating of metallic surfaces, usually iron surfaces, as a corrosion-preventing primer coating of metallic components or as an adhesive intermediate layer in the production of metal-elastomer compounds, for example, for vibration-damping components in the automotive industry. Autophoretic coating is thus a dip coating method which is performed without an external electric current, in contrast with electrodip coating, i.e., without applying an external voltage source. The self-precipitating compositions are usually aqueous dispersions of organic resins or polymers, which coagulate directly on the surface of the component in a thin liquid layer directly at the surface of the component on coming in contact with the metallic surface due to the pickling removal of metal cations and thereby determine the layer structure. The layer structure is self-limiting because the coverage of the metallic surface with the coagulated resin particles and/or polymer particles leads to a decline in metal dissolution, so that the coagulation process is delayed and ultimately terminated.
The technical implementation of the autophoretic deposition requires constant monitoring of the bath composition. First, the deposition bath must be adjusted so that a self-precipitation occurs rapidly enough and homogeneously enough on contact with the component, such that the stability of the dispersion in the interior of the dip bath must be permanently ensured at the same time. In addition, the concentration of the cations absorbed from the dip bath during the deposition process must not be allowed to rise above a bath-specific threshold level because otherwise the aqueous dispersion as a whole becomes unstable and coagulates.
The coatings deposited directly from autophoretic baths at first have a gelatinous consistency and therefore must additionally be converted to films. To this end, both organic resins and curing agents which crosslink with one another under thermal treatment are included in the self-precipitating compositions to thereby produce a closed polymeric coating, which protects against corrosion and can withstand mechanical stresses.
Both surfactant-stabilized dispersions and also polymer-stabilized dispersions which are suitable for providing a self-precipitating composition are known from the state of the art.
U.S. Pat. No. 7,037,385 discloses aqueous dispersions of an ionically modified phenolic resin, which can be adjusted to be self-precipitating by adding an acid, preferably phosphoric acid. The ionically modified phenolic resins disclosed there are self-dispersing. Such compositions should also impart an increased breaking strength to the coating in addition to providing corrosion protection of the coated component. The mechanical layer properties can be regulated according to the teaching of U.S. Pat. No. 7,037,385 by adding flexibilizing components based on polymer compounds, for example, polyacrylonitrile butadiene or polyacrylates.
However, U.S. Pat. No. 7,138,444 discloses compositions containing a dispersion of an epoxy resin obtained by emulsion polymerization of ethylenically unsaturated compounds in the presence of an epoxy precursor. For dispersion, mechanical methods are used with the aid of surface-active compounds, preferably anionically modified nonylphenols ethoxylates. Additional components include a curing agent based on blocked isocyanates as well as flow control agents and coagulation aids for thermal filming of the self-precipitating coating. In addition, U.S. Pat. No. 7,138,444 teaches that the solids content of the dispersion can be reduced if so-called accelerators consisting of fluoride ions and trivalent iron ions are added to a dispersion of the epoxy resin.
Additional additives typical of lacquer are usually added to autophoretic baths to impart certain physical properties to the fully hardened self-precipitating organic coating. The addition of colored pigments, especially black pigments based on carbon blacks, is a typical measure for visually opaque coloration of the organic self-precipitating coating on the metallic surface. However, practice has shown that such formulations often have only a low stability and the particulate components often sediment after only a short standing time of the autophoretic bath. However, agglomerates of carbon black particles together with the dispersed organic main binder can no longer be effectively deposited in a homogeneous and film-forming manner on a metallic surface introduced into self-precipitating compositions. Thus there is still a demand in the state of the art for extremely stable particulate compositions for autophoretic applications which contain, as colloidal organic binders, dispersed ingredients of an inorganic nature and/or which may be present in dispersed form as solid particles with another particle size distribution or surface charge in addition to the main organic binder. Likewise, in the automotive industry in particular, in the initial coating of metallic surfaces, the initial coating is required to not only laminate the metallic substrate but additionally to allow the application of top coats which impart color. For this purpose, however, only such initial coatings which reflect light mostly in the visible range and therefore contain a large amount of white pigment accordingly are allowed for such initial coatings. The addition of white pigments, for example, oxides of the metals titanium or zinc, with which those skilled in the art are familiar in the production of lacquer in an autophoretic composition does not usually yield stable particulate compositions because of the different manner of stabilization of the inorganic pigments and the dispersed main binder. For example, specifically dispersions of inorganic pigments based on oxides in the presence of polyvalent metal cations exhibit rapid agglomeration and therefore are very difficult to stabilize in self-precipitating compositions having a high iron(III) ion content. The stabilization of inorganic pigments is accomplished, for example, with certain wetting agents which may be ionic and nonionic surface-active compounds or water-soluble polymeric compounds. The specific wetting agents for inorganic pigments are therefore usually of a different chemical nature than those used for stabilization of the colloidal ingredients of the binder and for their use in self-precipitating compositions. Self-precipitating compositions usually have at least partial stabilization due to anionic surface-active substances. The difference in stabilization determines directly an exchange of the surface-active substances for the respective dispersed ingredients, so that agglomeration also occurs in such formulations and the bath stability exists only for a limited period of time.
In addition, for the complexing of the iron(III) ions, self-precipitating compositions also contain substantial quantities of fluoride ions, which in particular cause corrosion of inorganic pigments based on oxides and silicates and thus substantially reduce the pigment portion available for the autophoretic coating and thereby necessitate continuous addition of the pigment portion and fluoride ions to be able to establish the desired condition of the autophoretic bath. However, this is impossible, depending on the dissolution kinetics of the inorganic pigments due to the fluoride that is present.
In addition, the particulate components of autophoretic baths must be stable with respect to shearing forces which occur, for example, in circulation of the bath in the pump stands. For example, the commercial pigment preparations in which the particulate pigment portion is stabilized with wetting agents have a marked agglomeration tendency at those locations in the bath where high shear rates occur, as a component of autophoretic baths, because wetting agent is sheared away from the surface of the pigment particles.